SAW filters for Global Positioning Satellite (GPS) receivers

Interference, whether intentional (jamming) or unintentional, both in-band and out-of-band, can be a significant problem for receivers that operate in close proximity to high power transmitters. Out-of-band interference protection can be provided by high performance Radio Frequency (RF) filters used between the antenna and the receiver. These RF filters need to be low loss, have high out-of-band rejection and have nearly constant group delay to preserve the incoming signal characteristics. Well-designed superconducting filters can provide this level of performance and may also aid solutions for in-band interference, as the low loss of these devices allows their placement before the first Low-Noise Amplifier (LNA) in the system. We will summarize the results of experiments conducted with a set of electromagnetic HTS filters specially constructed for GPS frequency bands L1 and L2. We will also present recent data characterizing Lithium Niobate from room temperature to cryogenic temperatures. Although the temperature coefficient of delay of a Y-cut Z-propagating Lithium Niobate delay line is large at room temperature the coefficient of delay decreases as the temperature decreases until it becomes zero near 25K. Thus a combination of an ambient temperature quartz or Gallium Orthophosphate SAW bypass filter with either an HTS electromagnetic or an HTS Lithium Niobate SAW filter can provide linear phase filtering without impulse response ringing even in tactical applications.